Principles and Practices of Stud Welding - Imcyc Principles and... · Principles and Practices of Stud Welding Harry A ... 3.1 of ANSI/AWS ... moddkatmn based on physlcal and visual

Principles and Practicesof Stud Welding

Harry A. ChambersConsultantNelson Stud Welding, Inc.Elyria, Ohio

The embedment properties of stud welded anchors have been thesubject of many testing programs worldwide. Currently, designprovisions for cast-in-place anchorages are included in the 2000edition of the International Building Code (IBC 2000), and these willbe introduced along with design provisions for post-placed anchorsinto the 2002 edition of the ACI Building Code (AC1 3 18-02). In all ofthe testing programs, performance and failure modes of the as-welded studs with respect to groups, edge conditions, tension, shear,and combined loadings have been the central theme of the reports.Stud welding principles and practices necessary for obtainingconsistent stud weld quality and anchorage performance havereceived little attention. The purpose of this article is to present thefundamental principles of stud welding and implementing practicesso that the user may be confident in the ensuing welding results andperformance of the finished product.

E lectric arc stud welding was in-vented just prior to World WarII and developed out of a neces-

sity to attach wood planking to navalaircraft carriers. Threaded studs couldbe placed on the exterior side of steeldeck plates by using only one welderrather than drilling holes through theplate, a very slow and labor intensiveprocess. Since that time, the use of

studs has increased enormously in theappliance, automotive, and construc-tion industries.

Today, stud welding is widely usedin the construction industry. There aremany different stud welded productsthat are commonly used in the manu-facture of precast/prestressed compo-nents, including threaded, headed, anddeformed bars. These products are

PCI JOURNAL

welded to steel plates and other shapesthat are embedded in precast/pre-stressed concrete products for struc-tural connections (see Fig. l).’

The behavior of these embedmentsis critical to the performance of eachstructural element. Because thestrength and integrity of the finishedstructural system is vital to the overallsafety of the structure’s occupants andthe general public, engineers must givefull attention to the planning, design,and execution of these connections.

LITERATURE REVIEW

Test results that describe the perfor-mance of headed studs and deformedbars made. from cold deformed wire asembedments in concrete are availablein various publications.2-6 References 2and 3 describe the ability of studs todevelop full strength welds and dis-cuss the fact that, in some cases, somewelds were less than full strength.References 4 and 5 document embed-ment shear and tension tests of de-formed bar anchors where no weldfailures occurred. Reference 6 summa-rizes the results of extensive testingand studies from many sources on theperformance of stud welded anchorsand other types of anchorage devices.

STUD WELDING FAILURES

When the proper operation of studwelding equipment is combined withgood quality control and inspectionprocedures, full strength welds can beobtained consistently and result in op-timal performance of the embeddedanchorages. hi the author’s experienceand in discussions with other expertsin the stud welding industry, the rootcauses for weld or stud failures canusually be attributed to one or more ofthe following factors:

l Unacceptable base plate materialor plate surface condition

l Inappropriate weld settingsl Malfunctioning or obsolete

equipmentl Little or no formal training for

stud welding operatorsl Lack of quality control and in-

spection proceduresThis paper addresses these problems

by explaining the fundamentals of the

/ PrecastIPrestressed Tee

DeformedBar Studs

Strand Hold Down Stud

Fig. 1. Typical connection using deformed bar anchor studs in a precast/prestressedconcrete tee. Source: Nelson Stud Welding, Inc., Reference 1.

process and highlighting key issues inthe assurance of quality in the studwelding process. The first section dis-cusses basic principles, and the secondpart presents the recommended stepsfor producing quality stud welds andpreventing weld failures.

THE STUD WELDINGPROCESS

Electric arc stud welding involvesthe same electrical, mechanical, andmetallurgical principles found in any

other arc welding process. In studwelding, the power source and studwelding control system are set to con-trol the amperage and the arc durationor time. The welding gun has a trig-ger-activated circuit to initiate theweld and a lifting mechanism to drawthe stud away from the base materialand initiate the welding arc. The gunincludes a stud-holding chuck, twolegs, a foot piece, and a ferrule grip tohold the ceramic ferrule (also calledan arc shield) (see Fig. 2).

The sequence of operations for

Jot Grip

rig. L. 3wv weaving gun. 3vurm: Iwmvn xua vvelaing, Inc.

CHUCK

STUD

FERRULE

BASEMETAL

I W mI 1Fig. 3. Sequence of stud welding operations.A. A stud and ferrule are loaded into the gun, and the gun properly positioned against the base plate.. ,, ,B. The gun is pushed against the base material taking up the plunge, or stud length available tor burn ott against the gun spring

pressure. The trigger button is depressed to start the fully controlled automatic sequence. This sequence consists of initiating theweld current, and lifting the stud to create an arc by energizing the gun solenoid.

C. The arc duration time is completed and the stud plunged into the molten pool by means of a spring when the gun solenoid isde-energized and turning off the weld current at the end of the weld cycle.

D. The weld is completed and the gun is lifted off the stud and the ceramic ferrule removed. The stud is inspected for weld quality.

Fig. 4. Typical stud weltcross section. Source

Nelson Stud Welding, IncWeld Zones: A. Heat

unaffected stud materialB. Stud heat affected zone

(HAZ), C. Cast zoneD. Base material HAZ

E. Heat-unaffected bastmaterial

When performed properlythe stud weld is stronge

than both the stud materiaand base plate materialand failure will occur a

the ultimate steel strengtlin the stud shank or in thl

base plate, rather than ilthe weld

making a stud weld is illustrated inFig. 3.’ The fasteners for electric arcstud welding have a special shape andflux on the end of the stud that is to bewelded. This flux initiates and im-proves starting and stabilizing thewelding arc. It also serves to deoxidizethe molten weld metal for a sound.low-porosity weld zone to produce afull penetration weld.

The ceramic ferrule confines theweld arc and heat to a specific area ofthe base material and holds the moltenmetal in place to provide the uniformweld flash. The term weld flash isused instead of fillet because the weldzone comprises a mixture of meltedmaterial from the stud end and ex-pelled from the base plate material.rather than a weld made from a sepa-rate filler material that is deposited.

A completed weld cross section isshown in Fig. 4, with the various areasof the weld cross section identified.

STUD MATERIALSStud and base plate materials must

be compatible with the stud weldingprocess. Suppliers of both materialscan provide physical and chemicalcertification on the products they sup-ply and these should be requested onall orders as part of good quality con-trol recordkeeping. Studs of all styles

are available in low carbon steel andvarious grades of stainless steel. Com-monly used studs in the precast andconstruction industries are governedby the following American WeldingSociety (AWS) codes:

Low carbon steel studs:ANSYAWS Dl . l-00, Structural Weld-ing Code-Steel8 Studs are requiredto meet ASTM A 1O89 chemical speci-fications for Grades C-1010 throughC-1020 with the mechanical propertyrequirements listed in Table 1. Threestud classifications are identified asfollows (see Table 1):

l Type A studs are general purposestuds, of any type or size, usedfor purposes other than embed-ment or composite design con-struction. These include ‘/4 in.and 3/s in. (6 and 10 mm) headedanchors.

l Type B studs are headed, bent, or

some other configuration, iI2 in.(13 mm) or larger in diameter,used for concrete embedment oras an essential component incomposite design construction.

l Type C studs are ASTM A 496’Odeformed wires of any diameter,used for concrete embedmentpurposes.

Stainless steel studs: ANSUAWSD 1.6-99, Structural Welding Code-Stainless Steel.” Studs are required tomeet ASTM A 493” or ASTM A27613 chemical specifications forGrades XM-7, 304, 305, 309, 310, or316, or the low carbon versions ofthese alloys with the mechanical prop-erty requirements listed in Table 2.The properties listed apply to studTypes A and B as described in Table1. This code does not contain stainlesssteel Type C, deformed bar studs.

Acceptable base plate materials arelisted in ANSI/AWS D1.l, Table 3.1,Groups I and II. These include most ofthe common low carbon structuralsteels such as A36, A709, A992, andA516. Both the common carbon steelmaterials in Table 3.1 of ANSI/AWSDl .l and the stainless steel materialslisted in ANSUAWS D1.6, Section7.2.6, that meet ASTM A 276 are ac-ceptable base materials for stainlesssteel stud welds.

Table 1. Minimum mechanical property requirements for studs.6

Type A Type B Type CProperty AWS D1.l AWS D1.l ASTM A 4%5Tensile strength 61,000 psi 65,000 psi 80,OCKl psi(UTS) (420 MPa) (450 MPaj (552 MPa)Yield strength 49,000 psi 51,000 psi Not specified(0.2 percent offset) (340 MPa) (350 MPa)Yield strength Not specified Not specified 70,000 psi(0.5 percent offset)

Elongation (percent in 2 i n . )Elongation (percent in 5 x diameter)Percent area reduction

Note: 1 in. = 25.4 mm; 1 psi = 0 .006895 MPa.

17 201 4 15

50 percent 50 percent

(485 MPa)Not specifiedNot specifiedNoi specified

STUD WELDINGEQUIPMENT

Table 2. Minimum mechanicalproperty requirements for stainlesssteel studs.’ ’

Stud welding has been in continuoususe for over 60 years. Since the early19OOs, there have been many changesin the equipment used in the process,but the principles remain the same. Inthe basic process, DC power from aself-contained generator or trans-former/rectifier is passed through astud welding control system. The con-trol system is set at a determined time,voltage, and amperage to initiate thearc, which forms the molten pool. Theremaining unmelted stud shank is thenplunged into the molten metal pool.

Percent area reduction Not specified

Note : 1 m = 25 .4 mm; 1 ps i = 0 .006895 MPa.

into the power source, and transformerrectifiers are now the predominantsources of power (see Fig. 5). Systemsare available for welding studs with di-ameters ranging from l/s to 1 1/4 in. (3to 32 mm).

In the early years of stud welding, In recent years, the stud welding in-the power source was separated from dustry has been transformed from me-the control system. Power came from chanical to solid state welding equip-motor generators, battery banks, and ment with closed-loop controls thattransformer rectifiers. Today, the con- have contributed significantly to sim-trol system is typically incorporated pler weld setups and better weld qual-

Fig. 5. Transformer/rectifier powered stud welding systems. Source: Nelson StudWelding, Inc.

Table 3. Stud welding setups for mild and stainless steel studs welded to mild and stainless steel base materials.‘”

I/

Downhand weldig Overheadwelding Vertical welding

should a be considered so restrictive that they prevent moddkatmn based on physlcal and visual test evaluations of the finished welds as outlmed m AWS Dl 1 Structural Weld-ing Code-Stee l , AWS D1.6 S t ruc tura l Weldmg Code - Stamless S tee l and /or AWS C5.4 Recommended Prac t ices fo r S tud Weld ing , which would assist m finabzmp t h e meldsetup bawl on the condition5 and equipment at the productlon weldmg siteNote 2: 1 tn. = 25.4 mm; 1 sq m. = 645 mn?.

0.062 0.12J__m __J500 . 5 0 0 12.7 O.l!y!@ 800 0 . 5 5 __3? 0.062 0.125 800 0.062 0.125 875 0 . 4 7 2 8 ~ 0.062 0.125~.~0 . 6 2 5 15.9 0.3068 1200 0.67_ 40 0.093 0.187 40 ’ 0.093 -0.1870 . 7 5 0 19.1- 0.4418 lJQ0 0.84 50-55 0.093 0.1870 . 8 7 5 2 2 . 2 0.601_3 1 7 0 0 1.00 60-115 0.125 0.2501.000 2 5 . 4 0.7854 1 9 0 0 1 . 4 0 8 5 0.125 0.250 2050

/\lote 1: These welding parameters should be considered as an initial setup guide for the stud diameter and position being welded. They should produce satisfactory results, but

ity. These solid state controls provideverification measurement of both weldtime and current.

Current controls can adjust for weldcable resistance, compensate for in-coming power fluctuations, and pro-vide system shutdown in the event ofvariations from the set weld parame-ters. As another example, weldingguns with potentially erratic pneu-matic weld plunge dampeners havebeen upgraded to very reliable, self-contained, sealed hydraulic dampenercontrols.

Fig. 6. Typicatension tes

fixture(Reference 8)

STUD WELDING SETUP

An understanding of the settings andadjustments and their relationship withweld quality is needed to ensure con-sistent stud welding results. The fol-lowing definitions of specialized studwelding terms will make the narrativeeasier to understand.

l Plunge is the length of stud thatprotrudes beyond the ferrule. This por-tion of the stud is available to be“burned off,” or melted, to develop theweld fillet. A short plunge may cause

Slotted Fixtures toHold Stud Head andSpecimen Plate

excessive splatter and high or unevenfillet formation. Plunge is a physicalmeasurement set and measured withthe stud and ceramic ferrule in placeon the stud gun (see Fig. 2).

l Lift is the distance the gun pullsthe stud away from the base material.Before the weld is started, the stud andbase metal are in contact. Lift createsan air gap that the electric current mustbridge. The current flow across the re-sistance of this gap creates the arc heatto melt the stud and base material.

If no gap exists, the current will notcreate sufficient heat to fuse the metal.A short lift may allow the moltenmetal to bridge the arc gap, resultingin a cold weld. An excessively longlift increases the chance of having arcblow and welds that are bonded ononly one side of the fillet. Lift is phys-ically set on the stud gun and is mea-sured when the stud weld is initiated.Lift should be set and measured byplacing the stud and ferrule on a non-conductive surface and initiating theweld cycle so that an actual moltenweld is not made.

l Time is the duration of the weld.On thin base material, a shorter timeand higher amperage can be used toachieve sufficient heat and preventmelting through the base material. Onsome base materials, a longer time andlower amperage improve the ductilityof the weld zone. Weld time is set onthe time setting indicator of the con-trol system.

l Amperage is a measure of the cur-rent from the power source that flowsacross the air gap created by the lift.

Increasing the amperage increases theweld heat. As with the time setting, ahigher amperage setting is needed forlarger stud sizes. Amperage is also seton the control system’s current settingindicator.

l Alignment is the proper centeringthe stud in the ceramic ferrule so thatthe stud does not contact the ceramicferrule during lift and plunge, whichcan cause friction or binding betweenthe stud and ferrule. Binding can in-hibit both stud lift and stud plunge suf-ficiently so that there is less than suffi-cient stud melting and less than fullpenetration into the molten weld pool,resulting in a less than full strengthweld.

Table 3 contains suggested weld set-tings.14 These settings are suggestedstarting points for obtaining a finalsetup. The final setup is verified by vi-sual inspection, after weld measure-ments and physical testing to confirmweld quality. Other factors such aswelding system grounding, base platecomposition, and cable connectionscan influence the weld settings.

Stud welding manufacturers are re-quired by AWS codes8,” to qualifytheir standard stud weld base diame-ters by 60 stud tests of each stud diam-eter with the current at plus or minus10 percent from optimum [except inthe case of ‘/s and 1 in. (22 and 25mm) diameters, which are plus orminus 5 percent]. The tests requirebending and tensile testing all of thestuds to failure without failure in anyweld. Current range variations in thetest provide an indication of the ac-ceptability of the welds under condi-tions that can occur in actual shop orfield welding production.

Figs. 6, 7, and 8 illustrate typicalfixtures used for stud base tests, whichinclude the alternating 30-degree bendtest at a distance of 2 in. (51 mm)from the stud weld.*~” Fig. 9 showsacceptable and unacceptable weld fail-ures.*

STUD WELDING PRACTICEProper implementation of the prac-

tices presented in this section will re-sult in successful stud welding prod-ucts . As in other welding andfabricating methods, there are some

Double ActingHydraulic Cylinder

2 in. @mm)Maximum Angle of Centerline of

Deflected Stud Shall beMeasured at the CenterlineOf Plunger

Notes:1. Fixture holds specimen and stud is bent 30” alternately

in opposite directions.2. Load can be applied with hydraulic cylinder (shown)

of fixtureAdapted for use with tension test machine.

Fig. 7. Bend testing device (Reterence 8).

Stud Diameter7 p + l/64 in. (0.4mm)

Ffll- P i p e

Fig. 8. Suggested type of device for qualification testing of small studs (Reference 8).

Dimensions AppropriateFor Size of Stud

114 in.@mm)

Spec imen Plate

Fracture Line

Typical Fractures in Shank of Stud

I Fracture Line ,

Note: Fracture in weld near Note: Fracture through flash.stud. Fillet remains on plate. Fillet torn from plate.

Typical Weld Failures

Fig. 9. Typical acceptable stud failures and typical unacceptable weld tailures.

general guidelines to consider whendetermining the basics of good prac-tice. Among these are the followingconsiderations:

Weld Plate Thickness- A platethickness that is at least 0.33 times thestud shank diameter will develop thefull steel tensile and shear capacity ofthe stud. It is possible, however, thatthis ratio of stud diameter to platethickness will cause unacceptable dis-tortion and bending when high loadsare applied. A minimum plate thick-ness of one-half the stud diameter isoften specified and is recommended inthe PC1 Design Handbook.ls Platebending should always be a considera-tion in any embedment plate design.

Weld Plate Cleanliness- The areawhere the stud is to be welded (weldspot) should be as clean as possible.The spot where the ground clamp isfastened should also be cleaned onboth sides of the plate so that a goodcurrent path is established.

The presence of light rust or light

mill scale is not usually detrimental.Heavy mill scale or heavy, flaky rustshould be removed, as well as anydeleterious coating such as heavy oil,paint, galvanizing, grease, and mois-ture. Weld and ground spots can becleaned very quickly with an abrasivewheel, wire brush or wheel, drill burr,or end mill.

Note that solid grinding wheels orabrasive discs do not remove zinc gal-vanized plating very well. The grind-ing disc or wheel fills with zinc anddoes no more than spread the zincplating around, making the weld spotlook shiny and clean. Open pore abra-sive discs and grinding burrs are betteroptions for removing galvanizing.When zinc galvanizing is used to coatstuds, the coating should be removedfrom the weld end by the stud manu-facturer before the studs are shippedand welded.

Galvanizing- Even though zincgalvanizing is electrically conductive,studs should not be welded to a galva-

’ nized plate. Zinc is a contaminant thataffects the weld metallurgy, causingbrittle welds. Galvanizing should bedone after, rather than before, the baseplate has been welded to the stud.

The effects of hydrogen embrittle-ment should be considered when hotdip galvanizing is used. Hydrogen em-brittlement can have several points oforigin, which can cause serious brittle-ness in the weld stud shank whenstuds are bent to a very tight bend di-ameter. Causes of hydrogen embrittle-ment are improper pickling of the fin-ished weld plate, either by pickling toolong or not rinsing thoroughly afterpickling; plating too long or at toohigh a temperature; and using a studthat is of a much higher strength thanthe base plate material strength.

To prevent embrittlement in thebend area of bent bar studs, the bend-ing radius in studs to be zinc platedshould be four to six times the stud di-ameter as provided by the AC1 Build-ing Code.lh

Edge Distance- Studs should beplaced no closer to a base plate freeedge than the stud diameter plus ‘/* in.(3 mm) to the edge of the stud base.This distance should be at least 1 to 1 I/?in. (25 to 38 mm) from each free edge.

Grounding/Arc Blow- Edge dis-tance and ground placement can influ-ence weld quality due to arc blow: thatis, the welding arc is electromagneti-cally deflected toward the groundingpoint or toward the larger mass of thebase plate configuration being welded.Fig. 10 shows typical ground and edgeeffect patterns due to arc blow.”

These effects are less noticeablewhen large masses of steel are present.such as in large beams, but relativelysmall embedment plates can presentdifficulties. Typically, there is a lackof weld fillet on the periphery of thestud opposite the direction of the arcblow that can adversely affect weldstrength and quality.

As an example, a welding platentable may be grounded at all four cor-ners by four separate ground wiresbolted to the table and joined to thewelding system single ground wire bqa bolted connection. This turns the en-tire platen into a grounded mass andarc blow on a small plate set onto thetable is minimized. On longer, rectan-

gular plates, a double ground (one ateach end on opposite edges of theplate) provides a good current flowpattern.

Ground connections may be screwtype “C” clamps, fast action springclamps, or lever action hold-downclamps mounted to the welding tableand connected to the stud weldingsystem ground cables. All of thesetypes have proven successful. Fre-quently, a copper or steel plate largerthan the base plate to be welded, witha center ground bolted to the bottom,will eliminate or minimize the effectsof arc blow.

Weld splatter, weld berries, and fer-rule pieces should be removed fromthe weld plates so that the surface con-tacts the weld plate cleanly andevenly. This will ensure good electri-cal contact and current flow. It maytake some time and a few trials to es-tablish a good current flow path forthe typical base plate configuration.When proper grounding is establishedand arc blow is minimized, the resultis a consistent high quality weld with alow rate of re.jection and decreased re-pair costs.

Ceramic Ferrules- These piecesare also called ceramic arc shields. Fer-rules serve several functions. First,they contain the pool of molten metaland form it into the fillet (flash)formed around the periphery of thestud. Second, they control to a greatextent the amount of arc brightness andthe quantity of sparks expelled duringthe weld. Third, they are designed withspecific vent patterns so that when thearc is initiated, the flux in the stud endis consumed and deoxidizes the weldzone by expelling weld gasses throughthe vents, thus preventing oxygen fromentering the weld area.

Ferrules are also available in a widevariety of configurations to allowstuds to be welded to round tubing andbars, plate edges, channels, struts,rectangular and square structural tub-ing edges. These various configura-tions also allow for a number of weldpositions other than downhand.

It is important to keep the ferrulesdry. If they absorb too much moisture,the weld instantly turns the moistureinto steam with a substantial amountof molten metal expelled. In addition

a GROUND EFFEU b EDGE EFFECT

Fig. 10. Effect of (a) Ground connection, and (b) Workpiece on distortion ofmagnetic field (Reference 17).

to the possible danger from this “ex-plosion,” the weld is made veryporous and weak. Ferrules that havebeen wet or have absorbed moisturecan be dried by heating them to 250°F(12 1 “C) until the moisture is gone.

Ferrule cartons are marked with awarning that they contain silica, a pos-sible health hazard. Because the fer-rules are made from “green” fireclaywith binders and fired at high temper-atures, there is no free silica materialin respirable sizes released during theweld. If simply broken free of theweld, they are basically an inert, inor-ganic material, such as fired aggregateor rock, and may be disposed of easilyand safely. It would take an enormousamount of ferrule breakage or grind-ing to produce a dangerous health haz-ard to the stud welding operator orthose nearby.

Position Welding- Studs of allweld base configurations and diame-ters can be easily welded in the down-hand position. As a general rule, studsup to 3/4 in. (19 mm) in diameter canbe welded to the weld plate in a verti-cal position with consistent fullstrength results. Special ceramic fer-rules are used with studs s/8 in. (16mm) and larger when welding to thevertical position of the plate. There isa special ceramic ferrule for welding‘I8 in. (22 mm) diameter studs to thevertical position of the plate, but weld-ing this diameter with the base platevertical requires very carefully con-trolled conditions.

Welding overhead can also be done

with all stud diameters. Naturally, theoverhead position causes an increasedamount of welding sparks to fall dur-ing welding and suitable operator pro-tection is needed. There are spark re-tention accessories available from thestud manufacturer. Welding positionsare shown in Fig. 11 .‘I

Stainless Steel Studs Welded toCarbon Steel Base Plates- Fullstrength welds are made when stan-dard carbon steel studs are welded toeither approved stainless steel or car-bon steel base plate materials. Simi-larly, welds made with stainless steelstuds to stainless or carbon steel baseplate materials develop full stud steelcapacity in tension or shear. However.in cases where stainless steel studs arewelded to carbon steel plates and areto be subject to repetitive or cyclicloads, stress corrosion failure in theweld can occur, initiated by carbideprecipitation during the weld and sub-sequent intergranular cracking.

As an example, on one project,stainless threaded studs were weldedto low carbon structural steel to securecounterweight vibration dampers ex-posed to open weather conditions. Theconstant movement of the dampenerweights and repetitive loading cycleson the studs produced cracking, corro-sion and failure in the stud welds.

It is good practice to specify that thestainless studs to be used under suchconditions are either annealed aftermanufacture or made from annealedin-process stainless steel with lessthan 90 Rockwell B Hardness in the

Dowahand Vertical Overhead

is.

Limits of Positions For Plate or Pipe

4 s

Fig. 11. Positions tor stud welding (Keterence

finished condition. This minimizes thechance of weld cracking and failures.

Material Selection and Verification

Studs are made from steel suppliedto the stud manufacturer by quality ap-proved steel suppliers. Quality assur-ance procedures require that the steelsupplier provide certified mill test re-ports (CMTRs) for each heat and di-ameter of steel supplied. TheseCMTRs certify compliance to weldingcode specified material grades andchemistry. These reports are requestedat the time of purchase from the studmanufacturer and are part of their cer-tification package on every shipmento f s t u d s .

At the time of manufacture, studheats are also tested to determinewhether their mechanical propertiesare in compliance with welding coderequirements. A certificate of compli-ance (COC) for each stud shipment

can be made at the time of shipment toverify that chemical and physicalproperties comply with the specifica-tions of the engineer.

A COC can certify compliance withtypical steel specifications such asASTM A log9 and A 276,” as well asvarious welding codes such as AWSDl.l,* AWS D1.6,” CSA W.59,18 andIS0 13918.19 Similarly, the weld platefabricator should require certified milltest reports from their steel vendors toverify compliance to welding code-ap-proved materials.

As mentioned previously, weldingcodes require stud manufacturers toweld test and qualify their stud basediameters and materials. These testsmust be certified by an independenttesting laboratory, which will eitherconduct or witness the testing. Alongwith the stud COCs and CMTRs forboth studs and weld plates, the studweld base qualifications should bekept on file with the weld plate fabri-

cator to verify compliance with qualityprograms and welding codes.

Stud Welding at Low Tempera-t u r e - Studs should not be weldedwhen the base plate temperature isbelow 0°F (- 1 S°C) or when the surfaceof the base plate is wet, covered withfrost, or exposed to rain or snow.When the base plate is below 50°F(lO”C), impact testing of the weldedstud, such as with a hammer blow.should not be done. Instead, the studshould be tested by bending it slowlywith a hollow pipe or other bendingdevice. A brittle failure by impact test-ing at low temperatures in the weld orin the base metal is quite common.

Tension and shear tests done at tem-peratures as low as -40°F (-40°C) onstuds welded at -40°F have shown noloss in weld strength.20 It is the impacttesting that causes the weld failure.Whenever possible, welding and test-ing studs at low temperatures shouldbe avoided; if indeed it is necessary.the bend test should be done with apipe instead of a hammer.

Training and Qualification of the

Stud Welding Operator

The following section discussestraining of operators and process qual-ification.

Operator Training- Introductorytraining of operators in the stud weld-ing process is the first step in success-ful production stud welding. This fa-miliarizes the operators with thegeneral principles of the process.proper equipment setup, weld setupfor the studs, general guidelines. andinspection techniques. Trained and ex-perienced representatives of stud andequipment manufacturers can providespecifications, guidelines, and otherliterature as part of a complete pro-gram that will lead to formal qualifica-tion of both the operator and the studwelding process.

Process Qualification- Stud weld-ing is a prequalified process uniqueamong the many welding processesdue to the many millions of studs thathave been successfully welded usingthe process. When studs are welded inthe downhand position to approvedbase plate materials, only two studsare required to be welded and tested.

The test consists of both a visual andphysical inspection.

The physical inspection requiresbending the studs 30 degrees from thevertical by hammering the stud on theunwelded end or bending it with apipe or other device. The pipe shouldbe placed 2 in. (51 mm) above the studweld. The visual inspection verifiesthat the stud flash is complete aroundthe stud periphery, and that the after-weld height of the stud is appropriatefor the diameter being welded.

If the physical and visual inspectionare each satisfactory, then both theprocess and the operator are consid-ered qualified for those stud diameterswelded downhand. This qualificationapplies as long as there are no changesmade in the studs, settings, equipment,welding cables, or ceramic ferrules, orfrom an approved to a non-approvedbase material qualified by the test. Thetwo-stud test is required at the start ofevery production period, such as ashift change or operator change.

Studs welded to a non-approvedbase plate material or in a positionother than downhand, such as to avertical or overhead base plate, or thefillet or heel of an angle, must be ap-plication qualification approved.Qualification approval consists ofwelding ten of each stud style and di-ameter to be used in production to thepositions and base materials to beused in production, with the sameequipment, settings, welding cablesand ceramic ferrules to be used in theactual production welding.

All ten studs of each diameter and ineach position to be qualified must betested to failure by tensile, bend, ortorque test, or a combination of these.They must also meet visual inspectionrequirements. For all studs tested, fail-ure must be in the stud shank or baseplate material, rather than in the studweld. Note that failure is allowed inthe base plate material.

This provision is acceptable onlywhen it is known the base plate mate-rial may be of a composition, strengthor thickness that will not fully developthe stud weld strength, but the strengthand weld results are acceptable for theend use intended. Such a failure is notacceptable for embedment plates usedfor structural connections nor for at-

Fig. 12. Visual inspection of stud weld appearance. Source: Nelson Stud Welding, Inc.

tachments requiring full strength andductility. The successful completion ofthe specific application qualificationtests qualifies both the operator andthe process.

Qualification documentation for theoperator and the process should be partof the overall recordkeeping of the platefabrication facility. This documentationcan be completed by first establishing aWelding Procedure Specification(WPS) and Procedure QualificationRecord (PQR) for the stud diameters,equipment, settings, cable lengths,welding positions, weld plate materials,and ceramic ferrules that are to be usedin the weld plate fabrication.

Once the welding procedure specifi-cations are completed and docu-mented, the operators set up and per-

form the welding procedure using theWPS guidelines. The operator weldstwo studs of all diameters downhandand ten studs for all other positions.The welds are visually inspected andtested with the testing certified by thewelding supervisor or trainer. The op-erator is certified by name using thesame form as a Welder QualificationRecord (WQR).

These records are retained alongwith material certifications and pro-duction inspection records as part ofthe documentation files for review byinterested parties including customers,quality certification agencies, the engi-neer of record, and the owner’s repre-sentative. A sample WPS/PQR/WQRform for stud welding is provided inAWS Dl.l.*

Inspection Guidelines

Visual Weld Inspection- Aproper relationship between the lift,plunge, time, and amperage is neededto obtain good weld results. Thelength reduction or burn-off and theweld fillet appearance are determinedby the weld settings. Referring to Fig.12, visual weld inspection consists ofinterpreting the appearance of theweld fillet, and is normally a very ac-curate method if certain guidelines arefollowed.

A “good weld” is characterized by:l Even flash (fillet) formationl A shiny, bluish hue to the flash

surfacel A slight flow or bend of the flash

metal into the base materiall Good flash heightl Consistent after-weld lengthl Full “wetting” - i.e., flash around

the stud periphery without under-cut ,

A “cold weld,” which requires moretime, amperage, or both, is indicatedby:

l Low flash (fillet) heightl Incomplete flash formationl A dull gray cast to the flash sur-

facel Stringers of flash metal forming

“spider legs”

A “hot weld,” which is made withtoo much time, amperage, or both, isdistinguished by:

l Excessive splatterl A washed-out flash (fillet)l Undercutting of the studl Bum through the base material

“Stud hang-up” can result from toomuch lift, poor alignment of the studrelative to the work piece, and bindingduring lift and plunge caused bypoorly centering the stud in the ce-ramic ferrule. Stud hang-up is distin-guished by:

l Very low or no flash (fillet)l Severe undercut in the weldl No penetration of the weld into

the base platel Insufficient stud burn-off (after

weld height is too long)

Burn-Off - Stud Length Reduc-tion- In most arc welding processes,the weld fillet metal comes from astick electrode or a spool of welding

wire. In stud welding, a portion of thestud itself is the source of the filletmetal. The stud material that is meltedto develop the fillet is called bum-off.The difference in length between awelded stud and the original length isthe bum-off length.

The burn-off length is a very goodmeasure of weld quality, because theburn-off is determined by the weldsettings of time, current, lift, andplunge. Proper bum-off also indicatesthat there was no binding or hang-upduring the plunging motion of the gun.

The most convenient method ofchecking burn-off is to stand an un-welded stud upside down (load endup) next to a welded stud to comparethe length difference to the stud base -not to the end of the flux load. After-weld height can also be checked witha sliding carpenter’s level or square.Table 4 shows typical bum-off lengthreductions when welding to the bareplate.2’

Physical Weld Inspection- At thebeginning of the day, shift change, orany change of operator, equipment,position, or settings, two studs arewelded according to qualified settingsduring pre-production testing. Follow-ing satisfactory visual inspection, theyare bent 30 degrees (or torque tested inthe case of threaded studs). Studs thatare bent may be straightened to theoriginal axis and used in production.However, studs should not be heatedduring bending nor straightening with-out approval by the engineer of record.

Torque testing is performed to aproof load level slightly lower than thenominal stud yield to prevent perma-nent distortion of the threads. Torquetest proof load requirements are foundin AWS Dl .l or D1.6. The torque teststuds may be used in production.

The stud welding operator is respon-sible for pre-production setup and test-ing. The operator shall weld two studsto a production weld plate, or to apiece of material similar to the weldplate in material composition andwithin 25 percent of the productionweld plate thickness.

Inspections during production arealso the responsibility of the operator.Pre-production and production inspec-tion test results should be recordedand approved by the welding supervi-

Table 4. Stud burn-off lengths.*’

~1

Note: I in. = 25.4 mm.

sor. Unsatisfactory pre-production orproduction inspections and testsshould be brought to the welding su-pervisor’s attention and correctionsmade. This should be accompanied byadditional tests with fully satisfactoryinspection and test results before pro-ceeding with further welding.

At regular intervals during produc-tion welding, the studs welded sincethe previous testing interval should beinspected visually. If the visual in-spection shows a full periphery weldflash without undercut and satisfactoryafter-weld length. welding may con-tinue. If the visual inspection shows alack of fillet or insufficient weld bum-off, the questionable studs should bemarked and appropriate supervisorypersonnel notified.

In accordance with codes, the con-tract documents, or the quality assur-ance inspection criteria in use, thestuds without a full peripheral filletbut with a satisfactory after-weldlength may be tested by bending 15degrees in the direction opposite thelack of fillet, or repaired with a handweld by adding a minimum fillet weldas required in Table 5.8 The repairweld shall extend at least 3/s in. (10mm) beyond each end of the disconti-nuities being repaired.

If any tested stud fails the bend testor if there is continued and frequentevidence of insufficient stud bum-off.or incomplete fillet, production mustbe halted and appropriate supervisorypersonnel notified. The welding vari-ables should be checked, the necessaryadjustments made and the process andoperator qualification procedures re-peated with satisfactory results beforecontinuing with further productionwelding.

In many cases, the fabrication ofstud welded embedment plates may besubcontracted to an outside supplier.The weld plate fabricator’s inspectionprocedure and inspection resultsshould be made available to the pre-

Table 5. Minimum fillet weld sizes for studs.8*Stud diameter Minimum size fillet

i n .‘I4 through 7/,6 / 6.4through 11.1 % 5

\ ‘(1 \ Ylzn \ b% % ‘I8

\I,15.9, 19.0, 22.2 % 8

1 25.4 3h 1 0

Note: I tn. = 25.4 mm.* Welding shall be done with low hydrogen electrodes 5/3z or 3/,6 in. m diameter except that a smaller diameterelectrode may be used on studs ‘It6 in. diameter or under or for out-of-positma welds.

cast/prestressed producer. Further, theproducer should implement an incom-ing inspection procedure with accep-tance, rejection and corrective actioncriteria and corresponding records.

An excellent welding inspectionprocedure can be found in PCI’s Man-ual for Quality Control for Plants andProduction of Architectural PrecastConcrete Products.**

Equipment Service and Repair

The following section discussessafety precautions to be taken in theevent of malfunctioning equipmentand stud welding operations.

Malfunctioning/Obsolete Equip-ment and Maintenance- Older studwelding units with mechanical contac-tors and relays powered by a separatemotor generator or transformer recti-fier are obsolete. These systems lackread-out displays for time and current,current compensation, fault protection,and shut down, which allow controlledquality welds. Spare parts for thesesystems are not readily available.

Newer equipment is solid state, buteven a new system can suffer break-downs. Welding cable connections candeteriorate, wiring connections can be-come brittle and fail, and the accumu-lation of heavy dust or moisture cancause the solid-state components tooverheat and short out. Parts in thewelding gun can suffer wear or break-age, affecting the consistency of pro-grammed weld setup parameters.

It is good practice to regularly cleanthe welding systems and to inspectand replace worn or defective parts.Trained representatives of the equip-ment and stud supplier are usuallyavailable to assist in evaluating theequipment and cleaning guns or sug-

, gesting which components may needto be replaced.

Stud Welding Safety- As withany welding process, common senseshould be applied for the safety of thestud welding operator and welding sta-tion facilities. The welding area shouldbe kept clear of any flammable materi-als, gases, and falling and tripping haz-ards. The operator should also wearwelding s a f e t y g l a s s e s w i t hNo. 3 tinted lenses, protective gloves,welding aprons, and other protectiveclothing as needed for the setup of thewelding table and welding position.For example, a welding table at waistheight may subject the operator tosparks from the weld process, in whichcase a welding apron should be used.

Welding cables should be regularlyinspected for such things as loss of in-sulation and exposed connections.Any necessary repairs should be madewithout delay. Finished weld platesmust be handled carefully as they canbecome very hot during or shortlyafter welding is completed. The workarea should be adequately ventilatedso that welding fumes are exhaustedfrom the area.

Welding arc brightness is mini-mized due to the ceramic ferrule, butcontinual observation of the weldshould be avoided and tinted lensesare recommended. The sparks that arecaused by stud welding are usuallyminimal and do not carry for a longdistance, but workers should be atleast 5 ft (1.52 m) away from eachother. These safety practices and otherprecautions are outlined in ANSI/AWS 249.1 .23

CONCLUSIONS ANDRECOMMENDATIONS

Stud welding is a long recognizedand practiced welding method. In anygiven year and throughout the world,over 100 million stud welds of all

types are made with the requirementfor development of full weld strength.Quality welds are critical to the per-formance of finished structural mem-

bers, attachments, and connections.Whether the user employs the weldingprocedure in their in-plant fabricationshop or purchases the finished weldplates from a subcontractor, the princi-ples and practices discussed in this ar-ticle should be followed. The follow-ing summary recommendations aregiven for achieving consistent quality.

Stud and Weld Plate Materials

1. Studs should be manufacturedfrom materials acceptable for the studwelding process as approved in theAWS welding codes. The stud manu-facturer should be required, by specifi-cations from the purchaser, to providecertified material test reports on thematerial used to make the studs. Themanufacturer should also be requiredto provide certificates of complianceverifying that the shipped studs meetthe AWS codes and engineering speci-fication requirements provided by thepurchaser in the purchase documents.

2. Base plate materials should alsomeet the requirements for AWS codeapproved materials for use with studwelding and appropriate certificationsshould be required for the base materi-als from the steel manufacturer or theplate supplier. In both cases, the steelcertification documents are part of thefabricator’s quality control system andshould be maintained in their recordsand be included in the purchaser’s oruser’s documentation package.

Welding and Inspection

1. Stud welding training, equipmentsetup, welder qualification, and uniqueapplication qualification instructionare available from stud and equipmentsuppliers, and fabricators should availthemselves of these services. Therecords for such training can be incor-porated into an appropriate quality as-surance program.

2. Inspection of pre-productionwelds and continuous and docu-mented results of both visual andphysical inspection of finished weldsis further assurance of acceptableweld quality.

Stud Welding Equipmentand Safety

1. Equipment for stud welding hasgrown much more reliable than in pastyears, and improvements in the tech-nology are ongoing. Obsolete equip-ment may not function properly andshould be considered for replacement.All pieces of operating equipment are

subject to wear or possible failure.Equipment must be regularly inspected,repaired, or replaced as needed.

2. Welding safety for both operatorsand other personnel in the weldingarea is a matter of common sense.Publications are available with guide-lines for safety. Safety increases pro-ductivity.

ACKNOWLEDGMENTThe author wishes to express his ap-

ureciation to the PC1 JOURNAL re-I .viewers for their thoughtful and con-structive comments.

REFERENCES

1. Stud Welding Applications: Concrete Connections, NelsonStud Welding, Inc., Elyria, OH, 1988.

2 . Anderson, Neal S., and Meinhe i t , Dona ld F . , “Des ign Cr i te r iafor Headed Stud Groups in Shear: Part 1 - Shear Capacity andBack Edge Effects,” PC1 JOURNAL, V. 45, No. 5, September-October 2000, pp. 36-75.

3 . Strigel, Roberta M., Pincheira, Jose A., and Oliva, Michael G.,“Reliability of 3/8 in. Stud-Welded Deformed Bar AnchorsSubject to Tensile Loads,” PC1 JOURNAL, V. 45, No. 6,November-December 2000, pp. 72-82.

4 . Wiewel , Harry , “Load Capaci ty of Nelson D2L Deformed BarAnchors Ins ta l led in Concre te ,” Nelson Stud Welding, Inc . ,Elyr ia , OH, September 1986.

5 . Wiewel , Harry , “Repor t of Tes ts Conducted on Nelson D2LDeformed Bar Anchors Installed in Stone Aggregate Concrete,”Nelson Stud Welding, Inc., Elyria, OH, December 1994.

6. AC1 Commit tee 355, “State-of- the-Art Report on Anchorageto Concrete ,” AC1 355.1 R-9 1, American Concrete Inst i tute ,Farmington Hills, MI, 1991.

7 . Welding Handbook, Eighth Edition, V. 2, Welding Processes,Chapter 9 , Stud Welding, American Welding Society, Miami,FL, 1991, pp. 300-327.

8 . ANSYAWS Dl .l-00, Structural Welding Code-Steel, Ameri-can Welding Society, Miami, FL, 2000.

9 . ASTM A 108-99, “Standard Specif icat ion for Steel Bars , Car-bon, Cold Finished, Standard Qual i ty ,” American Society forTesting and Materials, West Conshohocken, PA, 1999.

10. ASTM A 496-97a, “Standard Specif icat ion for Steel Wire , De-formed, for Concrete Reinforcement,” American Society forTesting and Materials, West Conshohocken, PA, 1999.

11. ANSIIAWS D1.6-99, Structural Welding Code-StainlessSteel, American Welding Society, Miami, FL, 1999.

12. ASTM A 493-82a, “Standard Speci f ica t ion for Sta in less andHeat Res is t ing Stee l for Cold Heading and Cold Forging,”

American Society for Testing and Materials, West Con-shohocken, PA, 1999.

13. ASTM A 276-00, “Standard Specif icat ion for Stainless SteelBars and Shapes,” American Society for Test ing and Materi-als, West Conshohocken, PA, 2000.

14. Training Program, Nelson Stud Welding, Inc., Elyria. OH.2000.

15. PCI Design Handbook: Precast and Prestressed Concrete.Fifth Edition, MNL 120-99, Precast/Prestressed Concrete Insti-tute, Chicago, IL, 1999.

16. AC1 Committee 3 18, “Building Code Requirements for Struc-tural Concrete (AC1 318-99) and Commentary (AC1 3 18R-99),” American Concrete Institute, Farmington Hills, MI.1999.

17. Baeslack, III, W. A., Fayer, III, G., and Jackson, C. E.. “Qual-ity Control in Arc Stud Welding,” The Welding Journal.November 1975, pp . 789-797.

18. CSA W59-89, “Welded Stee l Cons t ruc t ion (Meta l Arc Weld-ing) ,” Canadian Standards Associa t ion , Rexdale, On ta r io .Canada, 1989.

19. IS0 13918:1998, “Welding-Studs and Ceramic Ferrules forArc Stud Welding,” In ternat ional Organizat ion for Standard-ization, Geneva, Switzerland, 1998, 21 pp.

20. Kennedy, D. J . Laurie , “Stud Welding at Low Temperatures ,”Canadian Journal of Civil Engineering, V. 7, 1980, pp. 442-4 5 4 .

21. ANSI/AWS C5.4-94, “Recommended Practices for StudWelding,” American Welding Society, Miami, FL, 1994.

22. Manual for Quality Control for Plants and Production of Ar-chitectural Precast Concrete Products, Third Edition, MNL117-96 , Precast/Prestressed Concre te Ins t i tu te , Chicago, IL.1996.

23. ANSYAWS 249.1-94, “Safe ty in Weld ing , Cut t ing and Al l i edProcesses ,” American Welding Society, Miami, FL, 1994.